There is known a magnetic random access memory (to be referred to as an MRAM, hereinafter) which stores data by controlling a magnetization direction of a memory cell. There are several kinds of MRAMs depending on a method for storing the magnetization direction.
In U.S. Pat. No. 6,545,906, a technique of a toggle type magnetic random access memory (to be referred to as a toggle MRAM, hereinafter) is disclosed. The toggle MRAM is a tunnel magneto-resistance element using a stacked ferric structure in a free layer of a memory element thereof. The MRAM has excellent selectivity of the memory cell at the time of a write operation, in which almost no multiple write is caused. The toggle MRAM disclosed in U.S. Pat. No. 6,545,906 will be described below.
FIG. 1 is a cross-sectional view showing a structure of a conventional toggle MRAM. A magneto-resistance element 105 in a memory cell 110 of this MRAM has an anti-ferromagnetic layer 104, a stacked ferric fixed layer 103, a tunnel insulating layer 102, and a stacked ferric free layer 101 which are laminated in this order. The stacked ferric fixed layer 103 has a stacked ferric structure, and includes a ferromagnetic layer 116, a non-magnetic layer 115, and a ferromagnetic layer 114. The stacked ferric free layer 101 has a stacked ferric structure, and includes a ferromagnetic layer 113, a non-magnetic layer 112, and a ferromagnetic layer 111. The magneto-resistance element 105 is put between a write word line 126 and a bit line 127 that are substantially orthogonal to each other.
The stacked ferric fixed layer 103 is formed as the stacked ferric structure to prevent leakage of a magnetic field from the stacked ferric fixed layer 103. A magnetization direction of the stacked ferric fixed layer 103 is fixed by the anti-ferromagnetic layer 104. The stacked ferric free layer 101 is also formed as the stacked ferric structure. The magnetic field is not leaked from the stacked ferric fixed layer 103 and the stacked ferric free layer 101 as long as no external magnetic field is applied.
FIG. 2 is a top view showing the structure of a conventional toggle MRAM. A plurality of write word lines 126 and a plurality of bit lines 127 are arranged to be orthogonal to each other (respective ones are shown in FIG. 2), and the magneto-resistance element 105 is arranged in each of intersections of them. The magneto-resistance element 105 has an easily magnetized direction (magnetization easy axis) which is directed at substantially 45 degrees (θ) relative to the word line 26 and the bit line 27. It is due to consideration for easiness of a toggle operation.
Next, a principle of a write operation in the conventional toggle MRAM shown in FIG. 1 will be described. In the toggle MRAM, data can be written only from “1” to “0” or from “0” to “1”. That is, it is impossible to overwrite data of “1” to “1” and “0” to “0”. Therefore, the write operation of the toggle MRAM is performed by reading data from a selected memory cell in advance and determining whether or not a magnetization direction should be changed or whether or not toggle operation is performed in first and second free layers on the basis of the read data and data to be written. That is, if the read data (“0” or “1”) is equal to the data to be written (“0” or “1”), the toggle operation is not performed, while the toggle operation is performed if a read data is different from the data to be written.
FIGS. 3A to 3H are diagrams showing a principle of the toggle operation in the toggle MRAM of a conventional technique as shown in FIG. 1. FIG. 3A is a timing chart of a write current IBL flowing through the bit line 127. FIG. 3B is a timing chart of a write current IWL flowing through the word line 126. FIG. 3C is a time change of a magnetization direction 121s of the ferromagnetic layer 113 and a magnetization direction 122s of the ferromagnetic layer 111 in a selected cell as the memory cell 110 into which data should be written. FIG. 3D shows a time change in a direction of a magnetic field generated by the write current IBL and the write current IWL. FIG. 3E shows a time change in a magnetization direction 121a of the ferromagnetic layer 113 and a magnetization direction 122a of the ferromagnetic layer 111 in each of non-selected cells connected to the same bit line 127 as the selected cell. FIG. 3F is a time change in a direction of the magnetic field generated by the write current IBL. FIG. 3G shows a time change in a magnetization direction 121b of the ferromagnetic layer 113 and a magnetization direction 122b of the ferromagnetic layer 111 in each of the non-selected cells connected to the same word line 126 as the selected cell. FIG. 3H shows a time change in a direction of the magnetic field generated by the write current IWL.
Referring to FIG. 3A, the write current IBL is provided for the bit line 127 at time t2 in the toggle operation. The write current IWL is provided for the word line 126 at time t3. The write current IBL is suspended at time t4. The write current IWL is suspended at time t5. Due to a series of the above current controls, a rotational magnetic field such as a magnetic field 123 to a magnetic field 124 to a magnetic field 125 as shown in FIG. 3D is added to the selected cell in a cross point between the selected word line 126 provided with the write current IWL, and the selected bit line 127 provided with the write current IBL. Therefore, the magnetization direction 121s of the ferromagnetic layer 113 and the magnetization direction 122s of the ferromagnetic layer 111 in the selected cell are rotated or changed as shown in FIG. 3C, so that data can be written. That is, data can be rewritten into a state of “1” if initial data has a state of “0”, and to a state of “0” if the initial data has a state of “1”.
At this time, a unidirectional magnetic field such as the magnetic field 123 as shown in FIG. 3F is exclusively added to the non-selected cell connected to the same bit line 127 as the selected cell. Therefore, the magnetization direction 121a of the ferromagnetic layer 113 and the magnetization direction 122a of the ferromagnetic layer 111 in the non-selected cell are returned to an original state with some fluctuations as shown in FIG. 3E, so that data is not written. Similarly, a unidirectional magnetic field such as the magnetic field 125 as shown in FIG. 3H is exclusively added to the non-selected cell connected to the same word line 126 as the selected cell. Therefore, the magnetization direction 121b of the ferromagnetic layer 113 and the magnetization direction 122b of the ferromagnetic layer 111 in the non-selected cell are returned to the original state with some fluctuations as shown in FIG. 3G, so that data is not written.
In case of the toggle MRAM, it is impossible to freely write “1” and “0”, so that data needs to be read once at the time of a write cycle, followed by being written thereafter. There is a demand for a technique to sort “1” and “0” to write while eliminating a multiple write.
FIG. 4 is a diagram showing a magnetic field applied to the magneto-resistance element in the selected cell. The magneto-resistance element 5 is inclined to a Y axis by θ. Therefore, magnetic fields HX (magnetic substance) and HY (magnetic substance) of the magneto-resistance element in the magnetization hard axial direction and a magnetization easy axial direction are inclined to the magnetic fields HX (wiring) and HY (wiring) of the wirings in an X axial direction and a Y axial direction by θ. It indicates a saturated magnetic field which is a maximum magnetic field to allow an anti-ferromagnetic coupling to be held between the ferromagnetic layer 113 and the ferromagnetic layer 111. A flop magnetic field is a minimum magnetic field required to reverse a magnetization direction of the stacked ferric free layer 101. An arrow indicates a route of a magnetic field applied to the selected cell.
In case of the toggle MRAM, composite magnetization of the stacked ferric free layer 101 approaches saturation in accordance with the increase of an applied magnetic field. In this case, there is a possibility of switching between magnetization of the ferromagnetic layer 111 in an upper layer and magnetization of the ferromagnetic layer 113 in a lower layer in the stacked ferric free layer 101 due to thermal disturbance. There is a demand for a technique which uses a write magnetic field to prevent composite magnetization from approaching saturation and to suppress a possibility of switching between magnetization of the ferromagnetic layer in an upper layer and magnetization of the ferromagnetic layer in a lower layer due to the thermal disturbance.
As a related technique, Japanese Laid Open Patent Application (JP-P2004-128237A) discloses a technique of a magneto-resistance effect element and a magnetic memory device. The magneto-resistance effect element includes a laminate structure which includes a free layer with the reversible magnetization direction, a fixed layer with a non-reversed magnetization direction, and an insulating film held therebetween. The fixed layer has a stacked ferric structure in which a first magnetic layer and a second magnetic layer are stacked via an intermediate non-magnetic layer, and a saturated magnetization of a material to form the first magnetic layer positioned on the insulating layer side is smaller than a saturated magnetization of a material to form the second magnetic layer.
Also, Japanese Laid Open Patent Application (JP-P2004-87870A) discloses a technique of a magneto-resistance effect element and a magnetic memory device. The magneto-resistance effect element includes a laminate structure which includes at least two ferromagnetic layers and an insulating layer held between the ferromagnetic layers. One of the ferromagnetic layers has a function as the free layer with a reversible magnetization direction, while the remaining ferromagnetic layer has a function as the fixed layer with a non-reversed magnetization direction. The free layer has a magnetic field application member to apply a magnetostatic field to the free layer.
Moreover, Japanese Laid Open Patent Application (JP-P2004-39757A) discloses a technique of a magneto-resistance effect element and a magnetic memory device. The magneto-resistance effect element has a pair of ferromagnetic layers to be mutually opposed via an intermediate layer, and is configured to obtain a magneto-resistance change by causing a current to flow perpendicularly to a film surface. One of the ferromagnetic layers is a magnetization fixed layer, and the remaining ferromagnetic layer is a magnetization free layer. The magnetization free layer is composed of a ferromagnetic material having an absolute value of a magnetostriction constant which is equal to or less than 1.5 ppm.
Furthermore, Japanese Laid Open Patent Application (JP-P2002-151758A) discloses a technique of a ferromagnetic tunnel magneto-resistance effect element, a magnetic memory, and a magneto-resistance effect type head. The ferromagnetic tunnel magneto-resistance effect element has a tunnel barrier layer which is formed between the first magnetic layer and a multi-layer structure of laminating at least five layers including a ferromagnetic layer and an intermediate layer. The first magnetic layer has a magnetization direction restricted to an external magnetic field. The ferromagnetic layer to compose the multi-layer structure has a magnetization direction rotated to an external magnetic field, in which the magnetization is anti-ferromagnetically arranged via the intermediate layer. The ferromagnetic tunnel magneto-resistance effect element has a ferromagnetic tunnel magneto-resistance effect film having resistance changed by a relative angle of the magnetization of the first ferromagnetic layer and the ferromagnetic layers to compose the multi-layer structure, lower and upper electrodes to be electrically connected to lower and upper magnetic layers in order to provide a sense current for the ferromagnetic tunnel magneto-resistance effect film, and a detection section adapted to detect a resistance change.